Abstract: In this paper, an improved method of estimating population density──" Synthetical threshold density method" is put forward through combining Iwao's zero frequency method with Gerrard's threshold density one,by which, the density estimation of adualt mites on apple tree and its sampling technique are approached. The mean density of adult mites is estimated by zero frequency method, and theoretical models of sampling numbers are obtained.It's shown that theoretical sampling numbers needed for zero frequency method are less than those for direct counting method.The fitting effect of synthetical threshold density method is better than that of Gerrard's.

Key words: Hawthorn spider mite, Estimation of population density, Sampling technique